Figure 1.

[Ca²⁺]_i dependence of the IP₃R in the presence of 10 μM IP₃. (a) Typical single-channel current traces of the IP₃R at various [Ca²⁺]_i. Arrows indicate closed-channel current level in all traces. (b) Dependence on [Ca²⁺]_i of mean closed-channel duration τ_c and open-channel duration τ_o of the IP₃R. (c) Dependence of IP₃R P_o on [Ca²⁺]_i. Experiments were performed with different Ca²⁺ chelators (□, BAPTA; ▵, 5,5′-dibromo BAPTA, ○, ATP) in the pipette solutions. Bath solution contained 1.5 μM Ca²⁺ and 0.5 mM ATP. Numerical labels indicate calculated concentrations (in μM) of free Ca²⁺ chelators present in the pipette solutions for the corresponding data points. The solid curve is the Hill equation fit for the biphasic [Ca²⁺]_i effect on IP₃R P_o. (d) IP₃R P_o vs. [Ca²⁺]_i in various bath solutions. Small open symbols: 0.8 μM luminal [Ca²⁺], 0.5 mM ATP; large open symbols: 0.2 μM luminal [Ca²⁺], 0 ATP; solid symbols: 0.2 μM luminal [Ca²⁺], 0.5 mM ATP. Convention for symbols and the solid curve are the same as in c.